In many digital data communications applications, a large number of end users communicate with a host computer over a packet switch network (PSN). For example, a large number of lottery ticket machines communicate with a host computer at lottery headquarters, transmitting lottery numbers and receiving acknowledgements that these numbers have been entered in the lottery. Another example is that of credit card verification, in which a large number of retail stores communicate with a host computer to determine whether purchases fall within their customers' credit limits.
In applications such as these, each end user is connected to the PSN by a trunk line. However, since a given end user needs to communicate with the host computer only occasionally, each trunk line to the PSN (and hence to the host computer) is frequently idle. Hence dedicating a trunk line to each end user is wasteful and costly.